The invention relates to a push-pull converter and a modulation method for controlling a push-pull converter.
Electric vehicles frequently have a device besides the drive system for charging the battery. Such a charging device facilitates in a simple way the charging of the electric energy storage of the electric vehicle at any power outlet. To this end, the charging device produces a DC voltage for charging the fraction battery from the public low voltage network.
An option for the embodiment of a charging device 10 for a vehicle with an electric drive is shown in FIG. 1. The charging device 10 has a power factor correction 2 (PFC) which is connected to an outer conductor 1a and a neutral conductor 1b of a public low voltage network. The power factor correction 2 is designed to extract sinusoidal currents from the supply network and to transfer said currents to a DC converter 4 via an intermediate circuit comprising an intermediate circuit capacitor 3 for smoothing the pulsating power extracted from the AC power supply. The DC converter 4 is designed to provide an appropriate DC voltage for charging the battery 5 of the vehicle with an electric drive.
For reasons of safety, a galvanic isolation between the public supply network and the vehicle is required for the charging device 10. To this end, the DC converter 4 is often embodied as a push-pull converter 4 in order to create said galvanic isolation.
An option for configuring a push-pull converter is schematically depicted in FIG. 2. The push-pull converter 4′ in FIG. 2 is a galvanically isolating, bidirectional DC converter comprising a so-called dual-active bridge topology. A full-bridge circuit consisting of the switching devices 131, 132, 133, 134 is thereby implemented in a primary-side input converter circuit, said switching devices transferring via a smoothing capacitor an input DC voltage to an output converter circuit by way of a transformer 139. The output converter circuit is likewise configured as a full-bridge circuit comprising the switching devices 135, 136, 137, 138 and can provide an output DC voltage 143 via a smoothing capacitor 141.
In order to control the push-pull converter 4′, different modulation methods exist, among others the trapezoidal modulation method, with which a battery charging capacity can be adjusted across a broad power range from approximately 400 W to 3300 W. In the trapezoidal modulation method, the current Ip takes on a trapezoidal waveform by means of the transformer 139, wherein a power transmission to the output converter circuit can take place in the plateau phases of the trapezoidal waveform as well as in a subsequent phase of a decreasing signal flank. The power to be transferred can thereby be adjusted via the height of the plateau.
Such push-pull converters 4′ and corresponding modulation methods for controlling the push-pull converters are, for example, described in the publications Krismer, F., Round, S., Kolar, J. W., “Performance Optimization of a High Current Dual Active Bridge with a Wide Operating Voltage Range”, “Proceedings of the 37th Power Electronics Specialists Conference, Jeju, Korea, Jun. 18-22, 2006” and Krismer, F., Biela, J., Kolar, J. W., “A Comparative Evaluation of Isolated Bi-directional DC/DC Converters with Wide Input and Output Voltage Range, Conference Record of the 2005 IEEE Industry Applications Conference, 40th IAS Annual Meeting (IAS'05), Hong Kong, Oct. 2-6, 2005”.
Power losses resulting from switching processes, which in each case are implemented in a half of the input or output bridge, can occur in the trapezoidal modulation method. A suitable and complex control method comprising time-critical current detection is thereby necessary to minimize the switching losses.